The invention relates to a precalendering method, in which a paper or paperboard web that has undergone earlier dewatering stages is calendered to attain a suitable surface for the coating process. The invention also relates to a finishing method for paper or paperboard. Furthermore, the invention also relates to an apparatus for implementing the methods.
In the papermaking process the dewatering stages include the initial dewatering that has taken place in the wire section for formation of a web, dewatering of the formed web by pressing in the press section and drying by means of heat in the drying section, in which the moisture of the web is reduced to a suitable level for further processing.
After the paper has been dried, desired surface structure of the web is attained by means of a mechanical treatment applied to the surface, i.e. calendering. There are many calendering methods, but it is characteristic to all of them that the web is brought through one or more nips, which is/are formed between two surfaces, typically between rotating roll surfaces. The purpose of calendering is to improve the paper quality by pressing the paper into a particular standard thickness and especially by smoothening its surface. In view of the coating to be applied to the paper at the next stage, the purpose of calendering is to bind loose particles on the surface and close the surface so that the coating layer would become even.
It is well known that by means of calendering it is possible to attain a desired quality, such as smoothness and gloss for the paper produced and processed at earlier stages. At the same time a fixed final density is attained for the paper. Thus, the calendering affects both the visual and structural properties of paper. In the technique according to the present application, calendering is conducted as a preliminary treatment especially with that purpose in mind that the desired properties are attained on the surface of the paper for the coating of the paper to be conducted at the next stage, and thus the visual properties of the base paper, such as gloss, are not significant.
Due to different machine concepts, one-sidedness occurs in the web going through the above-described dewatering stages, which can be caused for example by one-sidedness of dewatering and the different properties of surfaces located against the paper web. It is for example possible that one side is more closed and possibly rougher, and the other side is more open and possibly smoother. One-sidedness causes problems when paper is coated on both sides, and thus the aim is to make the paper surface suitable for the spreading of the coating composition by means of so-called precalendering before the coating operation.
The purpose of precalendering before the coating process is to improve especially the smoothness of the rougher side. By means of precalendering it is possible to eliminate one-sidedness in smoothness, but one-sidedness in absorption increases further, because after the treatment of more closed and rougher surface the surface becomes even more closed. In a film transfer coating process conducted by means of two rotating application rolls, in which the web precalendered in this way is coated on both sides, it is important to control the detachment of the web after a coating nip formed between the rolls. The aim is that the web would be detached from the lower roll last, because in this way it is possible to avoid runnability problems. Because many machine concepts cause such an effect that the upper side, which typically is placed against the upper application roll in the coating nip as well, remains in a more closed state, and the one-sidedness in absorption resulting therefrom cannot be corrected by means of precalendering even though the smoothness would be brought on the same level, the more open lower surface absorbs too much coating colour, whereas on the upper side more coating colour remains on the surface. Thus, due to the larger amount of coating colour, the upper side tends to follow the upper roll after the nip, and it is last released therefrom, which causes runnability problems.
One runnability problem that possibly occurs is formation of mist and this is solved by arranging auxiliary devices on the outlet side of the coating nip, as is disclosed in the Finnish patent 101092 and in the corresponding international publication WO 97/29239. In cases where mist formation takes place, the problems typically occur on that side from which the web is released last. If the web is last released from the lower roll, the fuming can be restricted to the lower side where it causes fewer problems.
Furthermore, as for the basic principle of a film transfer coater, reference is made to publication EP 608 206 A1.
In theory, the problem can be solved by using clearly unequal amounts of coating composition on different sides. Then, however, another problem occurs: the quality becomes one-sided, which is not a good aspect in printing papers.